Integration of surface tension and tissue pre-strain in alveolar cluster finite element model to predict lung...

The presence of surface tension forces associated with lung inflation are known to exist in vivo. In addition, lung tissue is subjected to pre-strain in situ at functional residual capacity. However, the contributions of these effects to the continuum-level mechanical response of lung tissue in deviatoric or non-volumetric deformations, important in human body models for safety, have not been quantified and are typically neglected in current lung models. In this study, an existing finite element micro-model of a representative alveolar cluster was enhanced with an explicit implementation of the surface tension membrane based on experimental data of surface tension forces in the lung. The micro-model was used to simulate, characterize, and quantify the effects of surface tension forces and alveolar pre-strains on the macroscopic mechanical response of lung parenchyma in uniaxial tension, compression, and pure shear. The results demonstrated that surface tension forces increase the stiffness of lung tissue in non-volumetric deformations, like how they increase the stiffness in volumetric deformations. The presence of tensile pre-strains in the alveolar wall that are present in vivo, further increase the stiffness of lung tissue in non-volumetric deformations relative to the stress-free lung condition. The combined effects of surface tension forces and alveolar pre-strains increased the model stiffness on the order of 10x in tension and 3x in compression and shear. The results presented herein indicate that the effects of alveolar pre-strain and surface tension are critical for continuum scale models of in vivo lungs to represent the mechanical properties of the lungs.